The present invention relates to a biaxially oriented transfer metallizing film based on polypropylene.
For certain packaging purposes, there exists a high, continuously growing demand for composite materials of paper and metal, in particular aluminum foil. Although extremely thin metal foils are already being used for such composite materials and satisfactory results are achieved using these foils, attempts have been made to save even more of the scarce and costly metal by the use of metallized papers.
The state of the art has disclosed two different methods of metallizing paper. In accordance with the first method, the metal is deposited directly on the paper under high vacuum, whereas according to the second method, the metal is first vacuum-deposited on another substrate and then transferred to the paper. The second process is also referred to as transfer metallizing.
For the direct vacuum-metallizing of paper, the choice of an appropriate paper quality is a decisive factor; furthermore, an expensive preparation of the paper is required. This preparation comprises sealing the paper by means of a varnish layer or subjecting the paper to a drying step, during which the normal water content of paper, that is, about 6 to 7%, is reduced to about 3%. Following vacuum-metallizing, the paper has to be readjusted to its normal water content. These procedures involve high expenditures of cost and time.
For transfer metallizing, a plastic film, usually based on a polyolefin or polyester, is provided with a metal coating. The metallized film is coated with a contact adhesive and bonded to the paper to be metallized, which in this case is selected in accordance with the requirements of the intended use. After the curing of the contact adhesive, the plastic film is peeled off from the composite structure, with the metal layer remaining on the paper. The process of transfer metallizing can be performed economically only if the plastic film can be reused several times. Conventional prior art films can be reused only up to about six times, and thus, there is a need in the art for a plastic film which can be reused to a greater extent.